Full-time working fathers out-earn their childless counterparts by more than a fifth, research suggests.

On average, fathers working full time get a 21% "wage bonus", the study based on 17,000 workers aged 42, concluded.

Fathers living in Britain with two children earned 9% more than those with just one, says the research by centre-left think tank IPPR for the TUC.

Full-time working mothers of the same age saw a "wage penalty", earning 11% less than their childless colleagues.

The report said the reasons for the "fatherhood bonus" were not clear, though they were likely to relate to hours worked, increased effort and positive discrimination.

It cited official labour market statistics showing full-time working men with dependent children worked on average half an hour longer each week than men without children.

It noted the same statistics showed that full-time working women with children worked about an hour less a week than those without children.

It also referred to a study which suggested there was a "long-term scarring effect" on the future earnings of women who initially take on part-time responsibilities when they return to work after maternity leave.

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Studies have consistently shown that men earn more than women

It said analysis based on fathers in the US found that increased work effort accounted for 16% of the fatherhood bonus.

The TUC said international studies cited in the report found that CVs from fathers were scored higher than identical ones from non-fathers.

In contrast, it said, CVs from mothers were marked down against those from childless women.

While mothers working full-time were on average found to have suffered a "wage penalty" compared with their childless female colleagues, older mothers were said to be paid 12% more. This was said to be because woman who gave birth when they were over 33 often had already developed their skills and were more likely to return to full-time work soon after maternity leave.

Current attitudes

It said the research reflected assumptions that fathers were the main breadwinners while mothers were expected to fit in work around looking after their children.

"It says much about current attitudes that men with children are seen as more committed by employers, while mothers are still often treated as liabilities," said TUC general secretary Frances O'Grady.

The research also addressed legislation surrounding parental leave.

"Financial considerations will be a major factor in decisions about who works and who cares," it said. "The low statutory pay available during shared parental leave will mean the highest earners - who are disproportionately male - go back to work by default, further entrenching gender gaps in employment, pay and representation."

The report used data from the 1970 British Cohort Study which follows the lives of more than 17,000 people born in England, Scotland and Wales in a single week of 1970.

Studies have consistently shown that men earn more than women, regardless of whether they are parents or not.

The Office for National Statistics (ONS) said in November that the gap between men and women's pay for full-time workers was 9.4% in April 2015, compared with 9.6% in 2014.